WWF's Living Planet Report sets bluefin tuna's UK comeback against a 73% average wildlife decline

The 73% averages the change in size of each tracked population. It is not a count of animals lost ^[3]. About half of the populations are stable or growing, and losses elsewhere outweigh their gains ^[4]. That second fact tells you something about how the average is built. No population can shrink by more than 100%. So if half sit at zero or above, a plain mean of their percentage changes could not show a fall much larger than 50% ^[20]. A 73% fall ^[1] has to come from combining or weighting the series some other way ^[20].

The index also covers only vertebrates. Dr Katie Collins, a senior curator at the Natural History Museum in London, said it misses plants, insects and other creatures that vertebrates depend on ^[5]. "If you lose garden snails, you lose the song thrush," she said ^[6].

The tuna is the report's clearest case, because the remedy matches the diagnosis. The collapse was blamed on overfishing ^[8]. The report credits the recovery to limits on fishing: lower catch quotas, size limits and protected spawning grounds, backed by campaigns from consumers, chefs and retailers ^[9]. Those measures act on the tuna itself, but the original decline also involved overfishing of the smaller fish it eats ^[8]. As reported, the evidence is a before-and-after record of one species ^[7][8]. The BBC's account of the report does not include a population figure for the fish. The scale of the rebound rests on descriptions: "huge numbers" off the British coast ^[7], and what Hannah Rudd, head of marine at the Angling Trust, called close to an "explosion" ^[10].

In my view the case supports Rudd's reading ^[11]. "I think bluefin tuna is a golden case study that fisheries management works," she said ^[11]. The measures target the stated cause, and the return followed them ^[8][9]. A single before-and-after case can show that much. A count would show how large the effect is.

The report's other recoveries took decades, or new law ^[13][14]. The green sea turtle moved from "endangered" to "least concern" last year after 50 years of conservation work in about 80 countries ^[13]. Elephants are stable or growing in parts of southern Africa, helped by laws that give local people a share of the income the animals bring, including from tourism ^[14]. The tiger counts imply a rise of about 78% over 15 years ^[21].

Mike Barrett, chief scientific adviser at WWF-UK, has worked on seven Living Planet reports and said this is the first whose takeaway is hope ^[16]. "Hope is not the same thing as wishful thinking," he said. "Hope is not passive. Hope's about action." ^[15]

WWF names farming and the burning of fossil fuels as the biggest pressures on wildlife ^[17]. Neither is among the levers credited with the tuna's return ^[9][17]. "So it's a closing window of opportunity, but it's still open, just, if we act now," Barrett said ^[19].